Based on the ligand selectivity in the receptor binding assay as seen in different tissues, the sigma receptor was found to consist of two subtypes, the sigma-1 and sigma-2 receptors. The incorrect early assumption that the sigma receptors would be members of the family of opioid receptors has been declined since the early 1990’s[1]. After many years of research, now it has been revealed that the sigma-1 receptor is a unique ligand-regulated molecular chaperone in the endoplasmic reticulum of cells[2]. Evidence was found that the most prominent action of sigma-1 receptors in biological systems is the regulation and modulation of voltage-regulated and ligand-gated ion channels, including Ca(2+)-, K(+)-, Na(+), Cl(-), and SK channels, and NMDA and IP3 receptors. Stimulation of the receptor by sigma-1 agonists causes inhibition of all above-mentioned voltage-gated ion channels, while on the other hand it potentiates ligand-gated channels[3]. In contrast, little is known to date of the sigma-2 receptor which is still to be cloned. Activation of the receptor seems to cause apoptosis[4].